I recently terminated employment and when I received my final pay check my accrued vacation time was not included.
HR stated that company policy

(To answer you question, the reason your final check does not include your accrued vacation pay is that our company policy regarding vacation, established under the Employee Retirement Income Security Act of 1974 ("ERISA") - which you signed when you started working here - states that:

"Up to five days of accrued vacation benefits will be paid at the time a full time employee resigns or is terminated if such employee has been with US for more than twelve continuous months.")

I was under the impression that vacation pay was entitled to you as wages eared and could not be taken away?

Is this correct or can I go after the lost vacation pay?

You are correct that “California” law mandates that employees must be paid the full amount of their accrued vacation upon termination. However, you mentioned that your company’s vacation plan was established under ERISA. This suggests to me that your company’s vacation payments are being made from a formally funded trust (like a Voluntary Employee Benefit Association – VEBA) If this is the case, “federal law” v. “California law” can govern the payment of vacation benefits on termination, and federal law, unlike California law, does not provide for the mandatory payments of one’s accrued, but unused vacation upon termination.